1. Field of the Invention
The present invention relates to a system for controlling the throttle opening of an automotive engine, and more particularly to a control system for angularly moving a throttle valve with a stepping motor to control the opening of the throttle valve.
2. Description of the Prior Art
Generally, a throttle valve and an accelerator pedal on an automobile are operatively interconnected by a link and a cable. When the accelerator pedal is depressed by the driver of the automobile, the throttle valve is actuated, and its opening is controlled by the accelerator pedal.
Another known throttle opening control system employs a stepping motor coupled to a throttle valve. The stepping motor is controlled depending on the depression of an accelerator pedal and engine operating conditions for throttle opening control. One such throttle opening control system is disclosed in Japanese laid-open patent publication No. 60-35141, for example.
According to the disclosed system, a control value is calculated at predetermined periodic intervals according to the accelerator pedal depression and engine operating conditions, and a number of command pulses for rotating the stepping motor are issued in each of the periods to actuate the throttle valve.
The disclosed system has a problem in that the timing to output command pulses for controlling the stepping motor is not definite, and a next command pulse may be outputted while the stepping motor is being rotated by previous command pulses. Usually, the present angular position of the stepping motor, i.e., the present throttle opening, can be detected by counting the number of command pulses that have been outputted. If a next command pulse is issued while the stepping motor is being rotated, however, it is difficult to accurately detect the present angular position of the stepping motor even by counting the number of outputted command pulses. To solve this problem, it is necessary to use a position detector and feed back detected positional information.
It has been proposed in Japanese laid-open patent publication No. 2-37133 to calculate command pulses to be applied to a stepping motor in each calculation period, limit the number of command pulses to a number that can be outputted in a next calculation period, and output the limited number of command pulses at an initial stage of the next calculation period.
The proposed process is effective to prevent a next command pulse from being outputted while the stepping motor is being energized, and allow the angular position of the stepping motor to be detected based on the outputted command pulses. Therefore, the throttle opening can be determined only based on the outputted command pulses, making a position detector unnecessary and simplifying the control system.
Stepping motors suffer mechanical resonance at a certain frequency due to their structures. As shown in FIG. 8 of the accompanying drawings, when the frequency f of command pulses applied to a stepping motor agrees with a resonant frequency fl, the stepping motor loses steps, and its torque T sharply drops. To prevent the stepping motor from losing steps, the interval of command pulses is selected such that the frequency f of the command pulses differs from the resonant frequency f1.
As described above, the interval of command pulses is selected such that the frequency f of the command pulses differs from the resonant frequency f1. In the case where command pulses are calculated in each calculation period and outputted at an initial stage of a next calculation period, however, no control is effected over the time duration from the completion of outputting of command pulses in a calculation period to the start of outputting of next command pulses in a next calculation period. The time duration is equal to the difference between the length of a calculation period and a time required to output command pulses within the calculation period, and varies with the number of command pulses. Therefore, the frequency corresponding to the time duration may agree with the resonant frequency of the stepping motor. When the frequency corresponding to the time duration agrees with the resonant frequency, the stepping motor may lose steps and hence not be controlled satisfactorily.